1. Field of the Invention
This invention relates generally to ghost cancelling systems for cancelling out a ghost signal at a video signal stage and more particularly is directed to a ghost cancelling system capable of immediately activating its operation when powered.
2. Description of the Prior Art
FIG. 1 schematically shows an example of a prior art ghost cancelling system. In FIG. 1, a video signal received by an antenna 1 is supplied through a tuner 2 and a video intermediate frequency (VIF) amplifier 3 to a video detector circuit 4 which then detects the video signal. This video signal is supplied through a delay circuit 5 with a delay time equal to an eliminating period of pre-ghost to a compounder or subtracter 6 which is also supplied with a ghost cancelling signal imitating the ghost and derived from a transversal filter as will be described hereinbelow and from which the video signal with ghosts cancelled out is delivered to an output terminal 7.
The video signal derived from the video detector circuit 4 is further supplied to a delay circuit 8 which forms a part of the transversal filter. In this delay circuit 8, delay elements, each of which takes a sampling period (for example, 10 nano seconds) as a unit of delay, are connected in plural stages (n number) to establish a delay time equal to the eliminating period of the pre-ghost and, n taps are led out from the respective stages. The signals from these n taps are respectively supplied to weighting circuits, 9.sub.1, 9.sub.2, . . . 9.sub.n each of which is formed of a multiplier.
The signal from the last stage of the delay circuit 8 is supplied to a terminal 10.sub.f of a mode switch 10, and the output signal from the compounder or subtracter 6 is supplied to the other terminal 10.sub.b of the mode switch 10. The signal derived from this mode switch 10 is supplied to a delay circuit 11. This delay circuit 11 is formed of delay elements, each delay element taking a sampling period as a unit delay time connected in plural stages (m number) to have a delay time equal to an eliminating period of delay ghost and m taps are led out from the respective stages thereof. The signals from these m taps are respectively supplied to weighting circuits, 12.sub.1, 12.sub.2, . . . 12.sub.m, each of which is formed of a multiplier.
The video signal from the compounder or subtracter 6 is supplied to a subtracting circuit 13. Further, the video signal from the delay circuit 5 is supplied to a synchronizing separator circuit 14 and the separated vertical synchronizing signal therefrom is fed through a standard wave forming circuit 15 to a low-pass filter 16 in which is formed a standard waveform approximate to a step-waveform of a rising edge VE of the vertical synchronizing signal. This standard waveform is supplied to the subtracting circuit 13.
The signal from this subtracting circuit 13 is supplied to a differentiation circuit 17 which then detects a ghost level of that signal.
As is well known in the prior art, for the ghost PG,4 level detecting signal, there is employed such a signal that is contained in a standard television signal and that is not affected by other signals during the period as long as possible, for example, the vertical synchronizing signal. That is, as shown in FIG. 2, during the periods between the rising edge VE of the vertical synchronizing signal and .+-.1/2H (H is the horizontal period) before and after the rising edge VE, the vertical synchronizing signal is not affected by other signals. Therefore, the aforesaid standard wave is subtracted from the signal in this period and the subtracted signal is subjected to the differentiation and thereby a weighting function is detected.
It is known in the art that when there is contained in the RF stage a ghost with the phase difference .phi. of 45.degree. from a desired signal and with a delay time .tau.(.phi.=.omega..sub.c .tau., where .omega..sub.c is the video carrier angular frequency in the high frequency stage), the ghost of the rising edge VE of the vertical synchronizing signal becomes such a waveform shown by reference letter VEG in FIG. 3A. Whereas, if this signal is differentiated and inverted in polarity, a ghost level detecting signal with a differentiation waveform shown by reference letter VEG' in FIG. 3B is provided. This differentiation waveform can approximately be regarded as an impulse response of the ghost.
Turning back to FIG. 1, the ghost level detecting signal of the differentiation waveform appearing from the differentiation circuit 17 is supplied through an amplifier 18 to demultiplexers 19 and 20 connected in series. The demultiplexers 19 and 20 each have such a construction similar to the delay circuits 8 and 11 in which delay elements, each of which takes a sampling period as a unit of delay time, are connected in plural stages and m and n taps are led out from the respective stages thereof. The outputs of these m and n taps are respectively supplied to switching circuits, 21.sub.1, 21.sub.2, . . . 21.sub.n and 22.sub.1, 22.sub.2, . . . 22.sub.m.
The vertical synchronizing signal from the synchronizing separator circuit 14 is fed to a gate pulse generator 23 which then generates a gate pulse having a pulse timing corresponding to a falling edge of the ghost detecting period and having an interval of, for example, 100 nano seconds. By this gate pulse, the switching circuits, 21.sub.1 to 22.sub.m are turned on, respectively.
The signals from these switching circuits, 21.sub.1 to 22.sub.m are respectively supplied to memory circuits, 24.sub.1, 24.sub.2, . . . 24.sub.n and 25.sub.1, 25.sub.2, . . . 25.sub.m, each of which is formed of an accumulative adder. The signals derived from the memory circuits, 24.sub.1 to 25.sub.m are respectively supplied to the weighting circuits, 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m.
The outputs of these weighting circuits, 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m are added together in an adding circuit 26 to form a ghost cancelling signal. This ghost cancelling signal is supplied to the compounder or subtracter 6.
As described above, the delay circuits 8 and 11, the weighting circuits, 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m and the adding circuit 26 constitute the transversal filter and thereby the ghost is cancelled out. In this case, even after the deformation of the waveform in the periods between the rising edge of a certain vertical synchronizing signal and the .+-.1/2H before and after the foregoing rising edge is detected and then a weighting function is decided, if there still remains a ghost, in order to detect such remaining ghost and to reduce the same, the memory circuits, 24.sub.1 to 25.sub.m are respectively formed of analog accumulative adders.
The on-and-off operation of the mode switch 10 enables the delay ghost cancelling circuit to be selectively changed from the feedforward mode to the feedback mode and vice versa.
While FIG. 1 shows such a case where a so-called output-adding type transversal filter is employed, FIG. 4 shows another prior art example which employs a so-called input-adding type transversal filter to cancel out a ghost. In FIG. 4, like parts corresponding to those of FIG. 1 are marked with the same references and they will not be described in detail.
As shown in FIG. 4, the video signal derived from the video detector circuit 4 is supplied to the weighting circuits, 9.sub.1 to 9.sub.n and the signals from these weighting circuits, 9.sub.1 to 9.sub.n are respectively supplied to input terminals of a delay circuit 8'. This delay circuit 8' consists of delay elements, each of which takes a sampling period as a unit connected in n stages and n input terminals provided at each stage between adjacent ones.
The signals at the input and output sides of the compounder or subtracter 6 are supplied to terminals, 10.sub.f ' and 10.sub.b ' of a mode switch 10'. The signal from this mode switch 10' is supplied to the weighting circuits, 12.sub.1 to 12.sub.m and the signals from these weighting circuits, 12.sub.1 to 12.sub.m are respectively supplied to input terminals of a delay circuit 11'. This delay circuit 11' consists of delay elements each delay element having a sampling period as a unit and the circuit 11' consisits of m stages and m input terminals provided at each stage between adjacent ones.
The signals respectively derived from the ends of these delay circuits 8' and 11' are added together in an adding circuit 26' to form a ghost cancelling signal. This ghost cancelling signal is supplied to the compounder or the subtracter 6.
Even with this circuit arrangement of FIG. 4, it is well-known in the prior art that, similarly to the ghost cancelling circuit employing the aforesaid output-adding type transversal filter, the ghost signal can be cancelled out, too.
Moreover, it is well-known in the prior art that, in the aforesaid ghost cancelling circuits shown in FIGS. 1 and 4, instead of the differentiation circuit 17, the differences between the neighboring two outputs of the memory circuits 24.sub.1 to 24.sub.n and 25.sub.1 to 25.sub.m are provided to produce difference outputs and then the difference outputs are respectively supplied to the weighting circuits, 9.sub.1 to 9.sub.n and 12.sub.1 to 12.sub.m.
Furthermore, it is also well-known in the prior art as, for example, disclosed in U.S. Pat. No. 4,357,631 that the demultiplexers 19 and 20 and the delay circuits 8 and 11 are made common and thereby utilized in time sharing manner.
As described above, it is well-known in the prior art that through the use of the transversal filter, the ghost signal can be cancelled out at the video signal stage.
By the way, in the conventional ghost cancelling circuit shown in FIG. 4, the switches 21 (21.sub.1 to 21.sub.n) and 22 (22.sub.1 to 22.sub.n), the analog accumulative adder-type memory circuits 24 (24.sub.1 to 24.sub.n) and 25 (25.sub.1 to 25.sub.n) and the weighting circuits 9 (9.sub.1 to 9.sub.n) and 12 (12.sub.1 to 12.sub.m) are arranged as, for example, shown in FIG. 5. In the figure, the signal from the demultiplexer 20 is supplied through transistors, Q.sub.1 and Q.sub.2 forming the switch 21 to the analog accumulative adder 24 consisting of a resistor R and a capacitor C. The voltage across this capacitor C is supplied through buffer-transistors, Q.sub.3 and Q.sub.4 to one transistor Q.sub.5 of a differential circuit which forms the weighting circuit (multiplier)9. The other transistor Q.sub.6 in this differential circuit is supplied with a predetermined fixed bias from a voltage source E. These transistors, Q.sub.5 and Q.sub.6 are respectively coupled to current paths of differential amplifiers, A.sub. 1 and A.sub.2. The video signals of opposite polarities are respectively supplied to the differential amplifiers, A.sub.1 and A.sub.2 and the outputs from these differential amplifiers, A.sub.1 and A.sub.2 are compounded and then supplied to the delay circuit 8'.
In this circuit arrangement, the ghost level detecting signals and the ghost cancelling or imitating signals are of a bipolar or positive and negative polarities. While, the demultiplexer 20 and the analog accumulative adder 24 are both circuits of a unipolar or single polarity. Therefore, in the aforesaid circuit, the demultiplexer 20 and the analog accumulative adder 24 supply a predetermined DC bias to the signal to process the signal in response to whether the signal is positive or negative polarity against this DC bias and this DC bias is subtracted from the signal by the transistors, Q.sub.5 and Q.sub.6 so as to carry out the weighting operation.
Now, let us consider the time when the power is applied to the aforesaid ghost cancelling circuit. Although the voltage source E is risen at the same instant the power source is turned on, the capacitor C of the analog accumulative adder 24 is charged for only 100 nanoseconds in, for example, one vertical period of the cycle when the switch 21 is made on, so that the capacitor C requires much time to reach the aforesaid DC bias. Thus, the weighting circuit 9 reaches such a state that it is apparently supplied with a negative weighting function signal of a large value to thereby perform erroneous ghost cancelling operation, taking much time to finally converge to the correct ghost cancelling operation. Also, during this period, the significantly deteriorated picture is reproduced.